Lipid composition-dependent membrane fragmentation and pore-forming mechanisms of membrane disruption by pexiganan
Dong-Kuk Lee1, Jeffrey R Brender, Michele F M Sciacca
1Departments of Biophysics and Chemistry, University of Michigan , Ann Arbor, Michigan 48109-1055, United States.
Abstract:
The potency and selectivity of many antimicrobial peptides (AMPs) are correlated with their ability to interact with and disrupt the bacterial cell membrane. In vitro experiments using model membranes have been used to determine the mechanism of membrane disruption of AMPs. Because the mechanism of action of an AMP depends on the ability of the model membrane to accurately mimic the cell membrane, it is important to understand the effect of membrane composition. Anionic lipids that are present in the outer membrane of prokaryotes but are less common in eukaryotic membranes are usually thought to be key for the bacterial selectivity of AMPs. We show by fluorescence measurements of peptide-induced membrane permeabilization that the presence of anionic lipids at high concentrations can actually inhibit membrane disruption by the AMP MSI-78 (pexiganan), a representative of a large class of highly cationic AMPs. Paramagnetic quenching studies suggest MSI-78 is in a surface-associated inactive mode in anionic sodium dodecyl sulfate micelles but is in a deeply buried and presumably more active mode in zwitterionic dodecylphosphocholine micelles. Furthermore, a switch in mechanism occurs with lipid composition. Membrane fragmentation with MSI-78 can be observed in mixed vesicles containing both anionic and zwitterionic lipids but not in vesicles composed of a single lipid of either type. These findings suggest membrane affinity and membrane permeabilization are not always correlated, and additional effects that may be more reflective of the actual cellular environment can be seen as the complexity of the model membranes is increased.
Insights
High concentrations of anionic lipids can inhibit antimicrobial peptides (AMPs) from disrupting bacterial membranes. Membrane composition significantly influences AMP mechanism, with mixed lipid environments promoting membrane fragmentation.
Area of Science:
- Biochemistry
- Membrane Biophysics
- Antimicrobial Peptides
Background:
- Antimicrobial peptides (AMPs) are crucial for innate immunity and exhibit potent antibacterial activity.
- AMPs' efficacy and selectivity often depend on their interaction with bacterial cell membranes.
- Model membrane systems are vital for elucidating AMP mechanisms, necessitating accurate representation of cellular lipid composition.
Purpose of the Study:
- To investigate the impact of anionic lipid concentration on the membrane disruption activity of the cationic AMP MSI-78 (pexiganan).
- To explore how varying membrane composition affects the mechanism of action and membrane interaction of MSI-78.
- To understand the correlation between membrane affinity and permeabilization in different model membrane systems.
Main Methods:
- Fluorescence-based assays to measure peptide-induced membrane permeabilization.
- Paramagnetic quenching studies using model micelles (anionic SDS and zwitterionic DPC) to probe peptide location.
- Experiments with single-component and mixed lipid vesicles to assess membrane fragmentation.
Main Results:
- High concentrations of anionic lipids inhibited membrane disruption by MSI-78, contrary to expectations.
- MSI-78 adopted a surface-associated inactive state in anionic micelles and a deeply buried active state in zwitterionic micelles.
- Membrane fragmentation was observed in mixed lipid vesicles but not in single-lipid vesicles, indicating a composition-dependent mechanism switch.
Conclusions:
- The bacterial selectivity of AMPs is not solely determined by anionic lipid interactions; high anionic lipid concentrations can be inhibitory.
- AMP membrane interaction and disruption mechanisms are complex and highly sensitive to lipid composition.
- Increasing model membrane complexity beyond simple lipid types reveals more biologically relevant mechanisms of AMP action.
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